A field changer device (hereafter called more simply “field changer”) generally comprises a rotary magazine in which several optical channels having different magnifications are provided.
Each optical channel has its own optical axis, which, by rotation of the magazine, is brought into coincidence with a fixed optical axis corresponding to the direction of observation.
In certain field changers (especially those with which microscopes are equipped), the magazine takes the form of a disk which is rotated about an axis coincident with its axis of symmetry, and on which disk several objectives are mounted, the individual optical axes of which are inclined to the rotation axis. Switching from one objective to its neighbor is performed by rotating the magazine through a predetermined angle corresponding to the angular difference between the projections, in a plane perpendicular to the rotation axis, of the individual optical axes of the objectives.
Owing to the various sizes of the objectives, the mass of such a changer is not balanced, the rotation of the magazine displacing the center of gravity of the field changer along a circle which is centered on the rotation axis, the relatively large diameter of which depends on the respective masses of the objectives.
This type of field changer is satisfactory for laboratory optical equipment, the choice of which is not determined by mass and balancing constraints.
However, this type of changer is especially unsuitable for optical equipment of large dimensions (for example in the field of telescopes), in which the masses of the optics are large and in which imbalances may result in alignment defects, making them more sensitive to vibratory environments that are often severe in the conditions of use of such equipment (shocks, vibrations, accelerations, etc.), or for air navigation assistance devices, in particular in the military field, where the onboard mass must be minimized and where the sighting precision of the changer depends on it being well-balanced.
Efforts have already been made toward better balancing of the changer. The reader may refer in particular to document FR 2 727 527, or to its equivalent U.S. Pat. No. 5,726,814, which describes a field changer having a cubic magazine, carrying three optical components mounted on faces of the cube and alternately brought into alignment with the optical axis by rotating the magazine through an angle of 120° about a diagonal of the cube.
Although this device marks real progress in terms of balancing, it does however have a number of drawbacks:                firstly, since each optical component occupies a pair of opposed faces of the cube, this can carry, at most, only three optical channels; and        secondly, the overall size of the magazine corresponds to a sphere, the diameter of which is equal to the length of its diagonal, that is to say about 1.7 times a side of the cube, which is necessarily greater than the diameter of the lens of the bulkiest optical component. In other words, the size of the magazine appears to be difficult to control.        